Description

During the implementation of my GSoC static_map competency project, I found that in Boost 1.63, string_view's comparison operator was not marked as constexpr(http://www.boost.org/doc/libs/1_63_0/boost/utility/string_view.hpp).
According to cppreference, string_view in C++17's standard library includes constexpr comparison operators, and actually the implementation of Boost's comparison operators calls constexpr compare() method. However, constexpr compare() in Boost never works in C++14, since it uses std::char_traits in standard library, of which char_traits::compare is not marked constexpr until C++17. It seems possible to backport C++17's constexpr char_traits into Boost's library.